An example of a structured signal receiver is a Global Positioning System (GPS) receiver which is part of the United States' Global Positioning System (GPS), also known as a radio-navigation satellite system (RNSS). The GPS was established by the United States government, and employs a constellation of 24 or more satellites in well-defined orbits at an altitude of approximately 26,500 km. These satellites continually transmit microwave L-band radio signals in two frequency bands centered at 1575.42 MHz and 1227.6 MHz, denoted L1 and L2, respectively. These signals include timing patterns relative to the satellites' onboard precision clocks (which are kept synchronized by ground stations) as well as navigation messages giving the precise orbital positions of the satellites, an ionosphere model, and other useful information. A GPS receiver processes these radio signals to compute ranges to the GPS satellites; and, by triangulating these ranges, the GPS receiver determines its position and its internal clock error.
GPS's designers assumed that all transmitters would be aboard satellites located at large and relatively constant distances from all user receivers, consequently generating weak but relatively constant signal levels at the receivers. This assumption drove a number of trade-offs in system and satellite transmitter design and continues to influence receiver development even today.
Despite this assumption, ground-based transmitters (known as PLs, pseudo-satellites, or simply pseudolites) have been used to complement the GPS satellites from the very beginning. In the foreseeable future, PLs may be incorporated in unmanned aerial vehicles (UAVs). A PL transmits a signal with code-phase, carrier-phase, and data components which may or may not have the same timing and format as the satellite signals. A GPS receiver acquires such a PL signal and derives code-phase pseudo-ranges or carrier-phase measurements to be used in a navigation algorithm in substantially the same manner as for a GPS satellite signal. The major differences between a satellite and PL are that a PL typically does not contain a high-accuracy atomic clock and that the PL position must be described using geographical terms rather than orbital elements.
Precision navigation and landing systems require reliable and highly accurate position, velocity and time (PVT) information that cannot be obtained by standalone GPS. Precision-guided weapons require reliable PVT information to achieve acceptable Circularly Error Probable (CEP) targeting errors. To meet these requirements, additional radio-navigation transmitters are needed. These transmitters can be additional satellites as specified in the Wide Area Augmentation System (WAAS); ground-based PLs as specified in the Local Area Augmentation System (LAAS); ship-based PLs; or, PLs on UAVs loitering in the air above an area of interest. WAAS and LAAS can transmit either correction data (i.e., differential data) or provide additional ranging information. When these transmitters broadcast augmentation signals in the GPS spectrum, additional interference is introduced into the GPS spectrum. This structured interference is like noise to the receiver, degrading the performance and in some cases preventing a receiver from acquiring and tracking the satellites.
Moreover, the use of PLs violates one of the key assumptions behind the design of GPS. The distance between a user receiver and a PL can range from short to long, so PL signal levels at a receiver can vary significantly. Relatively strong PL signals may overwhelm satellite signals and jam a receiver. Weak PL signals may be too feeble to allow receiver tracking. The challenge associated with this variable range effect is known as the “near-far problem” in wireless communications.
Equally problematic is structured interference arising from sharing the GPS radio frequency spectrum with other users or from the encroachment of other users' signals on the GPS spectrum. For example, Mobile Satellite Systems (MSS) downlinks, wind profiler radar, space-based radar, ultra-wideband systems, GPS expansion and the European Radio Navigation Satellite System known as Galileo, employ or may use frequencies in and around the GPS spectrum. In another example, GPS jamming technologies may broadcast interference signals in the GPS spectrum. These various RF systems introduce interference into existing RNSS systems either unintentionally or intentionally.
Another type of interference is self interference, which results when signals from a radio-navigation transmitter interfere with the reception of radio-navigation signals at the receiver. This type of interference often occurs when a RNSS receiver and transmitter are located physically near (or identical to) each other. Self interference is an extreme case of “near-far problem.”
In conclusion, many types of structured interference exist within the RNSS RF spectrum. It is desirable to have a method and apparatus to identify and remove such wireless interference that degrades or compromises legitimate radio navigation signals. In particular, it is advantageous to reduce or mitigate the near-far problem in radio navigation.